WIRE SELECTION HAS AN EFFECT ON PRIMARY STABILITY OF REVISION HIP STEMS

Abstract

Introduction: During hip stem revisions osteotomies allow to remove well-fixed components. Once removal has been done, cerclage wires should secure the osteotomy and support primary stability of the new stem. Stability is important for a bony ingrowth and therefore the longevity of a cementless revision stem.

Tension wires seem to dominate revision surgery and studies only refer to the advantages of cable wires in general. This in-vitro study analyzed the infiuence of both, tension and cable wires on primary stability of cementless revision stems. We aimed to examine the effectivity of wiring a femoral osteotomy, differences achieved with each method, and whether one wire has advantages regarding the fixation concepts of revision stems (meta- and diaphyseal).

Methods: We studied a Ti-tension- and a CoCrWNi-cable-wire. The Helios-stem stood for the meta- and the Wagner-SL-stem for the diaphyseal fixation concept. Each stem was implanted into 3 synthetic femurs and a standardized extended proximal femoral osteotomy was performed. Spatial movements of bones and stems at several sites were explored under axial torques using a high-resolution measuring device. Movement graphs subjected to the sites defined relative movements RM = ΔαZ/TZ [mdeg/Nm]. The osteotomies were locked consecutively with both wires and all compounds were measured again. Wiring was done by a proximal figure 8 and a diaphyseal circular loop.

Results: Compared to the unlocked osteotomy the tension as well as the cable wires caused a changed RM for the stems (p=0.03). Both wires affect an increased stability within the proximal main fixation area of the Helios. Even for the Wagner-SL, usually fixating diaphyseally, a proximal fixation was reached with both wires. A significantly better stabilization could be observed for the Helios using cable wires (p=0.04). The overall RM reached with tension and cable wires was 16.6 and 11.1 mdeg/Nm. The Wagner-SL^® showed no difference in stability between tension and cable wire treatment (p=0.29).

Discussion: Both, the tension and the cable wires support the revision stems in bridging the artificial defect of an extended proximal femoral osteotomy. Especially for the proximal fixating stem, RMs could largely be reduced, while cable wires seem to be advantageous. Preventing a circular constriction leading to an osseous malnutrition, the use of cable wires, however, should be impeded with regard to diaphyseal fixating stems and proximal osteotomies. Comparable results with both wires were reached and none of the wires showed any advantage in this situation. In conclusion, the wires should be chosen depending on the fixation concept of the revision stem.